The present invention relates to a process for removing selenium from waste water effluent, and more particularly, to a process for removing selenium from petroleum refining wastewater. The present invention also relates to a process for regenerating a fouled ion exchange resin used to treat selenium containing wastewater. Furthermore, the present invention relates to a process for recovering elemental selenium from selenide containing aqueous feed stream.
Selenium is a naturally occurring element that can occur in several oxidation states. It can exist in the [-II] (selenide), [O] (elemental selenium), [+IV] (selenite), and [+VI] (selenate) oxidation states, with only the latter two occurring in inorganic natural water systems. Selenium is a ubiquitous element having an average concentration of about 0.7 ppm in the earth's crust, and is concentrated in such diverse things as plants, sulfur deposits, sulfide minerals of copper and molybdenum, and fossil fuels. As a result, selenium can be found in waste streams from copper refining, acid coal mine drainage, coal-fired power plants, and petroleum refining.
Selenium has also been found in waste agricultural irrigation water. In this regard, the presence of selenium in waste agriculture and irrigation water recently has become a major pollution concern. Selenium poisoning has been implicated in waterfowl deaths and deformities at the Kesterson National Wildlife, Refuge near Los Banos, Calif. The potential of selenium poisoning, therefore, has generated significant interest in a number of different industries where research activity has been directed to the removal of selenium from wastewater.
Because the selenium concentration in waste streams is generally small, i.e., usually in the ppb range, wastewater treatment techniques have been of the adsorption type such as ion exchange, iron and aluminum hydroxide adsorption, and activated carbon adsorption. Also, lime precipitation and reverse osmosis techniques have been attempted.
Lime precipitation, however, has not achieved adequate technical success, while reverse osmosis has been found to be very costly. The success of the various adsorption methods depends largely on the selenium species present and on competitive ions in solution. As noted above, selenium can exist as selinide, elemental selenium, selenite, and selenate, with only selenite (Se[IV]) and selenate (Se[VI]) occurring in inorganic natural water systems. Of these species, ion exchange favors selenate over selenite, whereas the opposite is true for iron hydroxide adsorption. Since most natural waters include a mixture of the two selenium species, it has been difficult to approach complete removal using only one step. Furthermore, oxidation to, or reduction from, the selenate state is kinetically very slow which further inhibits optimization. Ion exchange also has not been a successful removal technique because selenate shows almost identical resin affinity as sulfate, which is usually present in a concentration of several orders of magnitude higher than selenate. Thus, the sulfate simply out competes selenium for resin sites. Furthermore, ion exchange resins become fouled when used to treat selenium wastewater and methods for regeneration are often inadequate and unpredictable.